As will be familiar to those skilled in the art, in a typical ion implanter a relatively small cross-section beam of dopant ions is scanned relate to a silicon wafer. Traditionally, a batch of wafers was mechanically scanned in two directions relative to a fixed direction ion beam.
With the advent of larger wafers, up to 300 mm in diameter, processing of a single wafer at a time has become advantageous in terms of cost, reduced wastage etc. Accordingly, it is now desirable to scan an ion beam relative to a silicon wafer by mechanically scanning the wafer in a first direction and electrostatically or electromagnetically scanning or fanning the ion beam in a second direction.
There are a number of different configurations of single wafer processing machines. One example is described in WO99/13488 and other configurations are described in U.S. Pat. Nos. 5,003,183 and 5,229,615. In WO99/13488, the wafer is mounted upon a substrate holder in a process chamber of an implantation device. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm and hence the substrate holder relative to the process chamber.
To facilitate movement of the moving parts of the scanning mechanism, one or more gas bearings are provided. For example, the end of the arm distal from the substrate support may be attached to a first bearing member which moves reciprocally relative to a second bearing member. This allows the wafer to be mechanically scanned in a plane orthogonal to the ion beam of the ion implanter. Movement of the first bearing member relative to the second bearing member is facilitated via a first gas bearing.
Likewise, the second bearing member may itself be rotatable relative to the process chamber to allow tilting of the substrate support relative to the direction of the ion beam. The second bearing member rotates against a stator mounted upon a flange adjacent the aperture in the wall of the process chamber; a second gas bearing is employed between the stator and the surface of the second bearing member to facilitate this rotation.
Since the process chamber is evacuated to a high vacuum and the exterior of the chamber is subject to atmospheric pressure, a large pressure differential exists across the second gas bearing. As is known in the art, in order to permit a vacuum to be maintained adjacent a gas bearing a series of differentially pumped channels are provided in one bearing surface. The vacuum which can be achieved in the process chamber depends on the gas flow leakage between adjacent channels. Thus, a greater vacuum can be achieved by increasing the distance between the channels. However, this also leads to an increase in the outside diameter of the bearing and greater vacuum forces on the movable part of the bearing.
It is an object of the present invention to address this problem. More generally, it is an object of the invention to provide an improved differentially pumped gas bearing vacuum seal assembly.